Hypervalent hydridosilicate in the Na-Si-H system.

Hydrogenation reactions at gigapascal pressures can yield hydrogen-rich materials with properties relating to superconductivity, ion conductivity, and hydrogen storage. Here, we investigated the ternary Na-Si-H system by computational structure prediction and synchrotron diffraction studies of reaction mixtures NaH-Si-H at 5-10 GPa. Structure prediction indicated the existence of various hypervalent hydridosilicate phases with compositions NaSiH (m = 1-3) at comparatively low pressures, 0-20 GPa. These ternary Na-Si-H phases share, as a common structural feature, octahedral SiH complexes which are condensed into chains for m = 1 and occur as isolated species for m = 2, 3. studies demonstrated the formation of the double salt Na[SiH]H (NaSiH, m = 3) containing both octahedral SiH moieties and hydridic H. Upon formation at elevated temperatures (>500°C), NaSiH attains a tetragonal structure (4/, = 2) which, during cooling, transforms to an orthorhombic polymorph (, = 4). Upon decompression, -NaSiH was retained to approx. 4.5 GPa, below which a further transition into a yet unknown polymorph occurred. NaSiH is a new representative of yet elusive hydridosilicate compounds. Its double salt nature and polymorphism are strongly reminiscent of fluorosilicates and germanates.